1. Field of the Invention
The present invention relates to a switching circuit integrated in a subterranean probe and, more particularly, to a switching circuit integrated in a subterranean probe for switching power propagated on a single wireline electrical conductor cable between two paths wherein each path of the two paths has coupled thereto at least a first probing means. More specifically, the present invention relates to a subterranean probe for performing a survey of a well borehole wherein such subterranean probe having said switching circuit integrated therein is switched between a surveying mode of operation and a location determination mode of operation for enhancing the validity of a survey of a well borehole by eliminating the measuring head error and the wireline stretch error. The present invention also relates to a method of switching the modes of operation of the subterranean probe.
2. General Background
A well borehole has a predetermined depth and an angle of inclination. For exemplary purposes, such depth is 20,000 feet and such angle of inclination is 45.degree.. The length of the well borehole is surveyed with a surveying probing means such as, without limitation, a rate gyro device. The rate gyro device, coupled to a wireline electrical conductor cable, is lowered into the well borehole to determine the azimuth of the well borehole at incremental positions along its length. The measurements by the rate gyro device must be correlated to the incremental positions of measurement. Therefore, prior to lowering the gyro rate device, a location determination probing means, such as without limitation, at least a gamma ray device, is lowered into the well borehole via a wireline conductor cable to calibrate the counters used to measure the length of the wireline as the gyro rate device is lowered to a predetermined depth in of the well borehole.
The gamma ray device uses known gamma ray charts in combination with sand, shale or collars charts to calibrate such counters, as is known in the art. Additionally, radio active markers could be used.
This process of surveying a well bore hole requires several hours to lower the gamma ray device to the depth desired for measuring the well borehole wherein such measurement patterns are correlated with the above identified charts to calibrate the counters. For example, to lower a gamma ray device 10,000 feet takes approximately 5 hours. After the calibration of the counters is complete, the gamma ray device is raised and replaced with the gyro rate device. Thereafter, the gyro rate device is lowered to the predetermined depth. Such predetermined depth may be different from the desired depth for the gamma ray device. The time to lower the gyro rate device is now 5 or more hours depending on the predetermined depth for the gyro rate device. As can be appreciated, integrating in the subterranean probe the location determination mode and the surveying mode and switching the subterranean probe between the two modes, at least 50% savings in rig time and man hours can be had.
This known method of surveying, using a gyro rate device and a gamma ray device, gives rise to errors in measurement and compromises the validity of the survey. Such errors in measurement are known as the measuring head error and the wireline stretch error. Such errors are the result of several factors. Such factors comprise, without limitation, (1) weight differentials between the gyro rate device and the gamma ray device since the counters were calibrated in relation to the weight of the gamma ray device; and (2) surface residue on the wireline conductor cable which results from the initial lowering into the well borehole. For example, if the surface of the wireline cable was dry when the calibration was performed, the wireline cable may become muddy, wet and/or exposed of a slippery polymer. Therefore, as the wireline cable is un-spooled to lower the gyro rate device in the well borehole, errors occur with respect to the measuring head coupled to the calibrated counters used to measure the depth of the gyro rate device. As can be appreciated, the measuring head error and wireline stretch error are eliminated when the surveying device and the location determination device are integrated into a single subterranean probe.
While various multiconductor wireline cables are readily available for delivering power to a plurality of probing means, the cost of the multiconductor wireline cable is at least twice the cost of a single conductor wireline cable. Since the wireline cable is tens of thousands of feet long, significant savings is had using a single wireline electrical conductor cable. Moreover, the time to un-spool (lower) and spool (raise) a single wireline electrical conductor cable is significantly less than the time to un-spool and spool a multiconductor wireline cable.
As can be seen, there is a continuing need for a switching circuit integrated in a subterranean probe for switching power propagated on a single wireline electrical conductor cable between two paths wherein each path of the two paths has coupled thereto at least a first probing means. Furthermore, there is a need for a subterranean probe for performing a survey of a well borehole wherein such subterranean probe having said switching circuit integrated therein is switched between a surveying mode of operation and a location determination mode of operation for enhancing the validity of a survey of a well borehole by eliminating the measuring head error and the wireline stretch error.